1. Field of the Invention
The present invention relates to lightweight plastic window elements generally used in aircraft, and particularly those that must be heated to provides means for dissipating fog or frost that forms on a surface of the window. The window usually comprises an outer window element comprising components of an acrylic resin that are fused together to form the element. One or more of the window elements may be a polycarbonate or a polyester. The window may also be provided with an electroconductive circuit that removes static electricity so as to provide a static dissipation circuit. A heating circuit for such a window comprises bus bars that are interconnected by a series of electroconductive wires that form the heating circuit. The wires forming the heating circuit are embedded in the plastic component in an array near its surface. Usually, the plastic component with embedded wires is fuse bonded under pressure with another plastic component to form a windshield element. The fused element may then be laminated under pressure to other windshield elements to form a laminated windshield.
The bus bars of the heating circuit are connected to a source of electrical power through terminal blocks and lead lines. One of the latter is grounded and one or more other lead lines connected to a power source. In the past, the electrical connections between the bus bars and the electroconductive wire of the electroconductive heating circuit were spot soldered. Such soldering required intense localized heating of the substrate which supported the electroconductive circuit and the bus bars. Localized heating caused localized stresses which interfered with the optical properties of the window and also rendered the windows less able to withstand stresses imposed during installation and those resulting from flexing and impacts during use in aircraft. In addition, it was difficult, if not impossible, to control the uniformity of thickness of the spaced solder points between the bus bars and the heating circuit. The thicker solder areas caused stresses that cracked the outer surface of the plastic window during further processing under pressure required for fuse bonding two or more plastic components to form a window element and/or laminating the window element so formed to other window elements.
Windshields with solder connections were failing during service. An examination of these windshields showed that the failure was accompanied by cracks that formed in outwardly facing plies of acrylic plastic in elongated regions overlying the bus bars. Inspection of the bus bar areas indicated thickness variations in the solder connections between the bus bars and the conductor wire because of uneven application of solder.
It is obvious that a technique for local electrical connections between an electroconductive wire heating circuit and a bus bar that avoids the need for spot soldering between the wires and the bus bars at spaced points therealong would be desirable in lightweight windows using heating circuits embedded in a plastic window element.
2. Description of the Prior Art
U.S. Pat. No. 2,813,960 to Egle and Bethge shows a laminated heated window in which heating wires are sewn or embedded in an organic interlayer material such as cellulose derivatives, polyvinyl butyral, polyamides or silicones or in ceramic materials, as well as glass, so that these materials can be generally used in area heating either in transparent or opaque bodies. The heating element is completely embedded in the insulated heating body material and is usually connected to a bus bar by spaced solder connections. The difficulty of controlling uniform solder thickness makes these units subject to the stress problem mentioned previously.
U.S. Pat. No. 3,383,762 to Leclercq discloses a laminated glass-plastic unit incorporating an electroconductive wire matrix embedded in a plastic interlayer. The ends of certain runs of wire are reversely looped around a bus bar. Spaced solder connections are applied to connect the reversely looped ends of the wire to the bus bar. The use of solder causes areas of increased thickness which cause localized stresses that would crack an acrylic plastic window element if the latter were substituted for glass in the Leclercq structure.
U.S. Pat. No. 3,947,618 to Gruss discloses a laminated glass plastic window that has a heating circuit that comprises wires extending between bus bars in random directions. The wires and the bus bars are embedded in a relatively flexible layer of interlayer material and the randomly directed wires have short length portions in the thickness direction as well as those extending parallel to the surfaces of the flexible layer. The wires are bonded between a segmented strip and relatively wide, continuous strip of bus bar material at random points as a result of lamination under pressure.
U.S. Pat. No. 4,078,107 to Bitterice and Keslar discloses a transparent plastic window containing embedded therein a static electricity dissipating circuit connected to a bus bar which leads to a grounded terminal and which may also contain a heating circuit insulated from the static electricity dissipating circuit and connected to a pair of bus bars suitably connected between ground and a source of voltage. The electroconductive wires of the various circuits are connected to the associated bus bars by applying two bus bar elements in the form of layers or strips of bus bar material, one above and one below the wires, and tacking the bus bar elements to one another and to the intermediate wires with a soldering iron. This soldering technique involves localized hot spots which establish very high local stresses. In addition, it is difficult to control uniformity of thickness in the spaced solder connections. The resulting areas of non-uniform thickness at the solder application points cause the plastic window element to develop cracks when the component containing the heating wires in the bus bars is fuse bonded to another component under pressure to form an element of an aircraft window and when the resulting element is laminated under pressure to other elements to complete the window. During service, these cracks developed into breaks and also caused optical and mechanical defects in the window.